The implosion which occurs upon breakage of the envelope of an evacuated cathode ray tube (CRT) is quite dangerous. Impact on the glass faceplate of such a tube can cause the faceplate to shatter into many fragments, which may be violently driven into the interior of the tube by external air pressure. The glass fragments then rebound outwardly and are ejected with sufficient force to cause serious injury to a person standing in front of the tube.
Until recently, all color television tubes have consisted of CRT,s with convexly curved faceplates. Such faceplates resist external air pressure in much the same manner as an arch supports an architectural load, and for that reason prior art methods of implosion protection have proved adequate. But curved faceplates require that the shadow mask employed in color TV systems must also be curved. Recently, a superior color CRT has been invented which employs a flat, tensioned shadow mask and a flat faceplate, and this has resulted in a major improvement in the brightness and/or contrast of the color image.
Unfortunately the implosion protection systems which have been used successfully with curved faceplate tubes have proven inadequate when used with flat faceplates. In particular, when prior art implosion protection systems are tested on the new flat tension mask tubes, they fail to meet UL1418, the relevant safety standard of Underwriters Laboratories, Inc. for television implosion hazards.
Three techniques of implosion protection are presently used with curved faceplates. In one of these, a metal band in hoop tension around the skirt (periphery) of the faceplate exerts a radial compressive force which cooperates with the external air pressure to put the curved faceplate under compression. This system is exemplified by the following U.S. Pat. No. Henry et al U.S. Pat. 2,847,017; Vincent et al. U.S. Pat. No. 2,785,820; and Lange et al. U.S. Pat. No. 3,200,188.
The tension band system described above depends upon the fact that the glass faceplate is under compression. Although brittle, glass is quite strong when it is under compression. The new flat faceplate, however, is bowed slightly inwardly by the effect of external air pressure. Therefore it is somewhat concave, which causes it to be under tension instead of compression, and makes it more vulnerable to breakage. Moreover, upon the occurrence of any rupture in the faceplate, its fragments tend to fly apart explosively because of the centripetal effect of the tension forces.
In another prior art system, known as the resin bond approach, a shell is placed around the faceplate skirt and filled with epoxy. The epoxy glues enough of the faceplate to the funnel (rear portion) of the tube to keep the scattering of glass fragments to a minimum.
Then there is a third approach, which involves securing an implosion protection panel to the front surface of the faceplate together of an adhesive which tightly bonds the two members together to form a monolithic structure. There is a significant body of prior art disclosing the use of bonded panels in connection with curved faceplates, including the following patents:
______________________________________ U. S. Patents Sumiyoshi et al. 4,031,553 Moulton et al. 2,596,863 Jackman 3,007,833 Giacchetti et al. 3,051,782 Hedler et al. 3,075,870 Kufrovich 3,113,347 Casciari 3,130,854 Anderson 3,184,327 McGary et al. 3,265,234 Applegath et al. 3,315,035 De Gier 3,422,298 Carlyle et al. 3,321,099 Lanciano 4,329,620 Arond et al. 3,208,902 Bayes et al. 3,177,090 Barnes 2,734,142 British Patents Downing 875,612 Darlaston et al. 889,457 ______________________________________
Attempts to use these prior art approaches with flat tension mask tubes have been unsuccessful. In particular, systems employing implosion protection panels tightly bonded to the front of the faceplate have not performed satisfactorily. High speed videotape motion pictures of test implosions of flat tension mask tubes with such bonded panels show clearly that the entire monolithic implosion-panel-and-faceplate structure disintegrates as a unit upon frontal impact, creating an abundant supply of glass fragments which are fired out the front of the tube at high velocity. The effect is a dangerous blizzard of glass shards.
It has now been discovered, however, that an improvement can be made in the bonded panel approach which dramatically reverses the results of the above-described experiments. This improvement consists in bonding the implosion panel to the faceplate in such a manner that the two will separate under impact. High speed videotape movies of flat tension mask CRT implosion tests, comparing the performance of such a system to that of prior art monolithic panel-faceplate structures, show an astonishing difference. No glass fragments escape the tube in the forward direction at all. The implosion panel survives the impact intact, and although the faceplate is cracked, its fragments are still in place. Subsequent inspection of the tube shows that the cracks have spread from the edge of the faceplate to the funnel of the tube, allowing ambient air to enter from the sides and thus equalize the pressure before the cracked faceplate can collapse under atmospheric pressure.
In addition ultra-violet-curable resin materials are used to bond the implosion panel to the outer surface of the faceplate. These resins permit curing by ultra-violet rays at ambient temperatures, without chemical curing agents, and in a much shorter period of time.
A preferred embodiment of the invention uses at least two layers of different UV-curable resin formulations applied to bond the implosion panel to the faceplate, the two formulations having substantially different levels of adhesion to glass to achieve separation of the implosion panel from the faceplate upon impact.
UV-cured resins have been used in the past to form plastic implosion-protection jackets for CRT faceplates; see British specification No. 889,457. But so far as is known, such resins have not been used to bond a separate implosion panel to such faceplates. Light-cured resins are used to bond two glass panes together in British specification No. 875,612; but there is no known suggestion of using ultra-violet curable materials in the CRT art.
In one specific embodiment of the invention, a first resin layer with a higher level of adhesion may be applied to the inner surface of the implosion panel, and a second resin layer with a lower level of adhesion may be applied to the outer surface of the faceplate, thus allowing the faceplate to separate from the implosion panel upon impact. U.S. Pat. No. 3,184,327 of Anderson employs multiple plastic layers for CRT implosion, and British specification No. 889,457 suggests using for the same purpose multiple plastic layers having different physical properties. But nowhere in the known prior art is there any suggestion that such multiple layers be used to bond an implosion panel to the CRT faceplate, nor any suggestion that the layers have differential adhesion with respect to such a panel and such a faceplate.
The following is a probable explanation for the dramatically improved results observed when the implosion protection system of this invention is employed. Upon frontal impact, the implosion panel and faceplate are both deflected inwardly. The relatively low level of adhesion between the resin bonding layer and the faceplate allows the latter to separate from the implosion panel. The thinner and more flexible implosion panel springs back, and the shock of impact is transferred through the more flexible implosion panel to the less flexible faceplate, which cracks as a result. The flexible resin layer cushions and blunts the impact to some extent. The implosion panel remains intact.
The flexure of the faceplate transfers high stresses to the skirt thereof, where the faceplate is secured to the funnel of the tube and therefore cannot readily flex. As a result, either the tube tends to fracture first in the vicinity of the faceplate skirt where the stress is highest, or if it cracks first at the point of impact, then the cracks quickly propagate to the faceplate skirt. In either case, the cracks tend to radiate quickly into the funnel portion, i.e. along the sides of the tubes, and are not confined to the faceplate. Consequently, atmospheric air enters the tube behind the faceplate and equalizes the pressure before the cracked faceplate can collapse. The faceplate fragments therefore remain in place. If some of them do escape, they will blocked by the still-intact implosion panel in front of the faceplate. The result is that no shards of glass are thrown outwardly.
It should also be noted that salvageability of an imperfect tube is enhanced by the implosion protection system of the present invention. Salvageability is of considerable importance because it permits manufacturers to reclaim a imperfect tube by disassembling it and saving the parts which can be reused. The differential adhesion system of the present invention permits the implosion panel to be easily removed from the faceplate by means of a wedge and mallet. The re-exposed front surface of the faceplate will be of virgin quality.